Method of producing malleable iron



United States Patent 3,511,721 METHOD OF PRODUCING MALLEABLE IRON Richard J. Light, Toledo, Ohio, assignor to Midland-Ross Corporation, Toledo, Ohio, a corporation of Ohio No Drawing. Filed Feb. 6, 1967, Ser. No. 614,022

Int. Cl. C21d 5/14 US. Cl. 148-138 1 Claim ABSTRACT OF THE DISCLOSURE This invention relates to a method of producing ferritic malleable iron from white cast iron and more particularly relates to an isothermal annealing cycle for the production of ferritic malleable iron. The inventive method disclosed herein provides a three-phase heat treatment for producing malleable iron from white cast iron wherein the cast iron is heated to a temperature of approximately 1750 F. and then fast cooled to a temperature of 1300 F., with a holding period at this temperature, and subsequently cooled to 1250 F., with a second hold period at this temperature. The iron is then cooled to ambient.

BACKGROUND- Over the past years, ferritic malleable iron has been processed in continuous furnaces using basic two-stage annealing cycle. The first stage consisted of heating the work to a temperature between 1700 F. and 1750' F. and holding at that temperature for a given period and the second stage was accomplished by rapidly cooling to approximately 1400" F. and then slowly cooling to 1250 F. The reason for the slow cool from 1400 F. to 1200 F. was to insure that all critical temperatures between these two points were held at a sufiiciently long time to catch all phase changes between these two temperature points. This slow cooling rate depended 'upon the composition of the work and varied from 5 F. to 30 F. per hour. The complete cycle was performed in a single furnace of considerable length, the length usually being in excess of 60 feet. The number of temperature zones in a furnace of this nature was in the order of 7 or 8, and resulted in the continuous furnaces being available only for large capacity foundries. It would he obviously advantageous to have a heat treating cycle that would lend itself to a furnace that did not require a plurality of heating zones and with a reduced length that could be placed in foundries having relatively small capacity.

OBJECTS It is, therefore, an object of this invention to provide a unique method for producing fem'tic malleable iron.

It is another object of this invention to provide isothermal annealing cycle for the production of ferritic malleable iron.

It is a further object of this invention to produce mallleable iron from white cast iron using a three-stage cyc e.

SUMMARY OF INVENTION Great strides have been made in the manufacture 01 white cast iron. It has been discovered that the modern methods of producing white' cast iron have resulted in cast iron having basically two critical temperatures at which the cast iron may be converted to ferritic malleable iron.

-These two basic temperatures are approximately 1300 3,511,721 Patented May 12, 1970 another heating zone at 1250 F.:25 F. for a sufficient length of time to equalize the temperature in the iron. After the time in the 1200 F. zone has passed, the iron is then placed in a zone at ambient temperature.

EXAMPLE In order to isothermally anneal white cast iron to produce malleable iron, a four zone furnace was used having a total length of 37 feet in to in brick. The first zone was set at 1700 F., zone number 2 was also set at 1700" F., zone 3 at 1300 F. and zone 4 at 1250 F. Work, consisting of white cast iron castings, was placed in the first zone for a period of eight hours, which proved sufficiently long to bring the work up to 1700 F., and then placed in the second or soaking zone for 24 hours to assure uniformity at the temperature of 1700 F. and complete the first stage graphitization. After the' heat had completely soaked through the work, it was then placed into the third zone at 1300 F. Because of the difference in temperature between thetwo zones, the drop in temperature was rather quick. The work was held in the third zone a sufiiciently long time to produce thermal equilibrium within the work and keep it at the critical temperature long enough to assure the complete phase change and partially complete second stage graphitization.

In this particular example, this took approximately 12 hours. The work was then placed in the' fourth zone, which was at 1250 F., and held therein sufficiently long to catch the second critical temperature change, and complete graphitization, this period of time being approximately 12 hours. After being in the last zone at 1250 F., the work was then placed in a chamber at ambient temperature.

Through the use of this inventive heat treat method, a relatively short furnace may be employed to produce' ferritic malleable iron. More particularly, the number of heat treat zones required is reduced substantially since' there is no more need to gradually change the work temperature. As a result, the cost of producing ferritic malleable iron is reduced and the capabilities of producing this type of iron is no longer limited to those foundries having the capacity to install furnaces of great length.

Although only a single embodiment of this invention has been shown and described, it is understood that changes and modifications can be made therein, and this description is illustrative only and not for the purpose of rendering this invention limited to the details illustrated or ,described except insofar as they are limited by the terms of the following claims.

I claim:

1. In an isothermal method of producing malleable iron from white cast iron, the steps comprising: placing white cast iron in a first environment at a first temperature between 1700 F. and 1750 F. for a sufficiently long time to reach thermal equilibrium and complete first stage graphitization; placing the iron in a second environment at a second temperature between 1275 F. and 1325 F. for a sufiiciently long time to reach thermal equilibrium and partially complete second stage graphitization; placing the iron in a third environment having a third temperature between 1225 F. through 1275 F. and holding at a sufiiciently long period of time to obtain thermal equilibrium and complete second stage graphitization.

References Cited UNITED STATES PATENTS 

